1. Background of the First Group of the Present Invention
The first group of the present invention relates to a method for drying coal by utilizing waste heat from a coal-fired boiler using coal (in particular, medium-quality or low-quality coal) as fuel, and to drying equipment therefor.
Medium-quality and low-quality coals are those types of coal which have a low degree of coalification, and typical examples thereof include high-volatile sub-bituminous coal and brown coal. Since they are found in abundance and most of them have a low sulfur content, it is believed that their use will increase in the future.
However, medium-quality and low-quality coals have a high moisture content. If they are burned in a highly moist state, a considerable heat loss results owing to the sensible heat of the moisture and its latent heat of vaporization. Accordingly, the calorific value per unit weight is so low as to cause an increase in transport cost. Moreover, they have the disadvantage of being highly hygroscopic and liable to spontaneous combustion. These problems have constituted an obstacle to the expansion of their utilization.
Thus, there has been employed a method for drying such medium-quality or low-quality coal by heating it to a temperature of 80 to 150° C. at the colliery (or the site of coal mining) (this method is also called “ordinary drying”). Most of the dried medium-quality and low-quality coals have been consumed at the colliery for purposes of electric power generation and the like, while only a small portion thereof has been sold and transported to consumers in its neighborhood, and used thereby. In short, it has not been common practice to transport or store dried coal derived from medium-quality or low-quality coal, because this has a risk of spontaneous combustion.
Generally, in coal-fired boiler equipment, air heated by an air heater is introduced into a coal pulverizer where coal is dried and pulverized. The resulting pulverized coal is pneumatically conveyed to the burner and burned as boiler fuel.
2. Background of the Second Group of the Present Invention
The second group of the present invention relates to coal having low spontaneous combustibility, low hygroscopicity and a high calorific value which is obtained by heating medium-quality or low-quality coal to a high temperature, cooling it, and then subjecting the resulting reformed coal to an aging treatment, and to a method for producing the same.
Medium-quality and low-quality coals are those types of coal which have a low degree of coalification, and typical examples thereof include brown coal and high-volatile sub-bituminous coal. Usually, coals having a rank equal to or lower than sub-bituminous coal are called medium-quality and low-quality coals. Since they are found in abundance and most of them have a low sulfur content, it is believed that their use will increase in the future. However, medium-quality and low-quality coals have a high moisture content and hence a low calorific value per unit weight. Moreover, when dried, medium-quality and low-quality coals have the disadvantage of being highly hygroscopic and liable to spontaneous combustion. These problems have constituted an obstacle to the expansion of their utilization.
Accordingly, as described previously, there has been employed a method for drying such medium-quality or low-quality coal at a temperature of 80 to 150° C. (this method is also called “ordinary drying”). However, when coal is dried according to this method, the resulting dried coal shows an increase in spontaneous combustibility and, moreover, still has high hygroscopicity. Consequently, this dried coal tends to absorb atmospheric moisture during transportation and storage, and return to its original state freed of surface moisture alone (i.e., its original internal moisture content before drying). Moreover, owing to its high spontaneous combustibility, this dried coal is unsuitable for transportation and storage and has been limited to use at the site of coal mining (e.g., for purposes of electric power generation).
Moreover, in order to evaporate moisture, decompose hydrophilic oxygen-containing groups, and prevent the volatilization of tar, another method wherein coal is dried at a temperature of 180 to less than 300° C. is known. However, as contrasted with the reformed coal produced by the above-described method comprising rapid heating to a high temperature and subsequent cooling, the reformed coal produced by this method fails to show a sufficient improvement in calorific value and long-term spontaneous combustibility.
Furthermore, Japanese Patent Publication No. 11596/'82 discloses still another method for producing reformed coal by heating medium-quality or low-quality coal rapidly to a temperature of 300 to 500° C. and then cooling it rapidly. This method brings about a substantial improvement in spontaneous combustibility and a marked increase in calorific value due to dewatering, but fails to achieve sufficient stability of spontaneous combustion properties. Consequently, in order to prevent the spontaneous combustion of the resulting coal, a prudent measure needs to be taken under general consideration.
Thus, even if any of the conventional methods is employed, it has been difficult to produce reformed coal having desirable properties such as low spontaneous combustibility, low hygroscopicity and a high calorific value, from medium-quality or low-quality coal.
3. Background of the Third Group of the Present Invention
The third group of the present invention relates to a process for producing reformed coal having low hygroscopicity, a high calorific value and low spontaneous combustibility by heating medium-quality or low-quality coal to a high temperature and then cooling it, wherein the reformed coal is produced by utilizing hot exhaust gas from a coal-fired boiler and equipment for treating such exhaust gas.
Since medium-quality and low-quality coals are found in abundance and most of them have a low sulfur content, it is believed that their use will increase in the future. However, medium-quality and low-quality coals have a high moisture content and hence a low calorific value per unit weight. Moreover, when dried, medium-quality and low-quality coals have the disadvantage of being liable to spontaneous combustion and highly hygroscopic. These problems have constituted an obstacle to the expansion of their utilization.
Accordingly, as described previously, Japanese Patent Publication No. 11596/'82 discloses a method for producing reformed coal by heating medium-quality or low-quality coal rapidly to a temperature of 300 to 500° C. and then cooling it rapidly. Moreover, Japanese Patent Laid-Open No. 70093/'81 discloses a method for producing reformed coal by heating medium-quality or low-quality coal rapidly to a temperature of 300 to 500° C. and then cooling it rapidly, wherein the raw coal is dried by using a gaseous mixture composed of hot air obtained by subjecting air to heat exchange with the exhaust gas having been used for the rapid heating purpose and part of the hot gas for rapid heating use, so that the reformed coal can be advantageously produced from the viewpoint of thermal efficiency. Furthermore, as described previously, still another method for producing reformed coal is known. In this method, coal is dried at a temperature of 180 to less than 300° C. in order to evaporate moisture, decompose hydrophilic oxygen-containing groups, and prevent the volatilization of tar.
In any of the above-described methods for producing reformed coal having low hygroscopicity, a high calorific value and low spontaneous combustibility from medium-quality or low-quality coal, combustion gas from an installed gas is used and, moreover, a cooling gas is used as a diluent gas for controlling the temperature of the combustion gas. However, this cannot be regarded as economical from the viewpoint of equipment construction and thermal efficiency.